Universität Stuttgart
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Item Open Access Confirmation of siderazot, Fe3N1.33, the only terrestrial nitride mineral(2021) Bette, Sebastian; Theye, Thomas; Bernhardt, Heinz-Jürgen; Clark, William P.; Niewa, RainerSiderazot, the only terrestrial nitride mineral, was reported only once in 1876 to occur as coating on volcanic rocks in a fumarolic environment from Mt. Etna and, to date, has been neither confirmed nor structurally characterized. We have studied the holotype sample from the Natural History Museum, London, UK, originally collected by O. Silvestri in 1874, and present siderazot with epsilon-Fe3N-type crystal structure and composition of Fe3N1.33(7) according to crystal structure Rietveld refinements, in good agreement with electron microprobe analyses. Crystal structure data, chemical composition, and Raman and reflectance measurements are reported. Possible formation conditions are derived from composition and phase stability data according to synthetic samples.Item Open Access Electronic structure and defect chemistry in iron perovskites(2021) Hoedl, Maximilian F.; Maier, Joachim (Prof. Dr.)This thesis systematically investigates the electronic structure and defect chemistry of BaxSr1-xFeO3-d through first-principles density functional theory (DFT) calculations. First, the electronic structure of defect-free, cubic BaFeO3 was calculated using DFT and analyzed in terms of local atomic orbitals. The calculations revealed BaFeO3 to be a negative charge transfer material with a dominating d5L (L = ligand hole) configuration. A detailed chemical bonding analysis further showed that the Fe-O bond has a mixed ionic-covalent character, and that the frontier orbitals at the Fermi level (and ligand holes) have an anti-bonding pdsigma* character. The susceptibility of the ideal cubic perovskite structure towards phase transformations was evaluated on the basis of first-principles phonon calculations. The phonon dispersion revealed distinct dynamically unstable modes which are isostructural to Jahn-Teller type distortions. The distortion is able to lift the orbital degeneracy of O 2p dominated ligand holes inherent to the cubic phase, thereby alleviating stresses in the electronic structure. The defect chemistry of BaxSr1-xFeO3-d was explored with respect to two different types of point defects: oxygen vacancies and protonic defects. The energy of oxygen vacancy formation, i.e. the release of neutral oxygen at the expense of electron holes, increases with increasing Sr-content and increasing oxygen vacancy concentration. Both compositional variations correlate with an increasing Fermi level at which electrons from the removed oxygen have to be accommodated. With increasing oxygen vacancy concentration, the Fe-O bond is weakened which facilitates oxygen excorporation and should decrease the vacancy formation energy. However, this contribution is effectively outweighed by the concomitant increase in Fermi level, rendering the vacancy formation energy to experience a net increase. In solid oxides containing oxygen vacancies, protons can be incorporated via the hydration reaction, i.e. the absorption of water vapor in dissociated form (H+, OH-), with the proton being attached to a regular oxygen ion and the hydroxide ion filling an oxygen vacancy. A thermodynamic formalism was developed that allows quantifying the energy changes during the two partial reactions - the proton- and hydroxide affinities - from first-principles DFT calculations. The new formalism was applied to a wide range of solid oxides, ranging from binary oxides such as MgO to various perovskite oxides, including BaZrO3 and BaFeO3. The study revealed an intriguing correlation between proton- and hydroxide affinities and the ionization potential (IP, position of O 2p band relative to the vacuum level) of the materials across the various structure families investigated. In the series of compositions BaxSr1-xFeO3-d, the hydration energy becomes more negative with increasing Ba-content and increasing concentration of oxygen vacancies. Evaluation of the proton and hydroxide affinities in oxygen non-stoichiometric BaFeO3-d showed that the trend with oxygen vacancy concentration largely reflects an underlying trend of increasingly more negative hydroxide affinities. This is suggested to stem from the annihilation of delocalized ligand holes during oxygen vacancy formation; lattice oxygen ions (and incorporated OH-) become subsequently more negatively charged, and thus experience a stronger electrostatic interaction with their ionic environment.Item Open Access Character of doped holes in Nd1-xSrxNiO2(2021) Plienbumrung, Tharathep; Schmid, Michael Thobias; Daghofer, Maria; Oleś, Andrzej M.We investigate charge distribution in the recently discovered high-𝑇𝑐 superconductors, layered nickelates. With increasing value of charge-transfer energy, we observe the expected crossover from the cuprate to the local triplet regime upon hole doping. We find that the 𝑑-𝑝 Coulomb interaction 𝑈𝑑𝑝 makes Zhang-Rice singlets less favorable, while the amplitude of local triplets at Ni ions is enhanced. By investigating the effective two-band model with orbitals of 𝑥2-𝑦2 and s symmetries we show that antiferromagnetic interactions dominate for electron doping. The screened interactions for the s band suggest the importance of rare-earth atoms in superconducting nickelates.Item Open Access Electrical conductivity in quantum materials(2021) Mitscherling, Johannes; Metzner, Walter (Prof. Dr.)Item Open Access Correlated phenomena in graphene at high and low carrier density(2021) Geurs, Johannes; Smet, Jurgen H. (Dr.)This dissertation studies electronic transport in graphene at high and low carrier density. At very high carrier densities, the band structure of graphene features a Van Hove singularity. Magnetotransport measurements suggest that below 110K, a pseudogap opens in graphene, combined with a Lifshitz transition. At low carrier density, it is possible to treat the electron liquid in graphene by electron hydrodynamics. A graphene Tesla valve is presented, a device that works as an electron viscometer.Item Open Access Low-energy optical conductivity of TaP : comparison of theory and experiment(2021) Yaresko, Alexander; Pronin, Artem V.The ab-plane optical conductivity of the Weyl semimetal TaP is calculated from the band structure and compared to the experimental data. The overall agreement between theory and experiment is found to be best when the Fermi level is slightly (20 to 60 meV) shifted upwards in the calculations. This confirms a small unintentional doping of TaP, reported earlier, and allows a natural explanation of the strong low-energy (50 meV) peak seen in the experimental ab-plane optical conductivity: this peak originates from transitions between the almost parallel non-degenerate electronic bands split by spin-orbit coupling. The temperature evolution of the peak can be reasonably well reproduce by calculations using an analog of the Mott formula.Item Open Access Miscibility, viscoelastic reinforcement, and transport properties of blend membranes based on sulfonated poly(phenylene sulfone)s(2021) Saatkamp, Torben; Maier, Joachim (Prof. Dr.)Chemical energy that hydrogen may generate during combustion and the corresponding electrical energy are interconvertible by means of a fuel cell (FC) and by the electrolysis of water (WE), which allows for the utilization of the complementary nature of these two key energy vectors towards energy sustainability. A proton exchange membrane (PEM) made from an ionomer is commonly employed as the electrolyte in mobile fuel cell applications and in water electrolyzers that require dynamic operability and pressurized product gases. New PEM materials are needed to increase performance, reduce environmental impact, and allow for a more targeted design of PEMFC and PEMWE systems, all of which is in some way limited by the use of the established perfluorosulfonic acid (PFSA) type ionomers. This work’s focus lies on sulfonated poly(phenylene sulfone)s (sPPS), a unique group of fluorine-free cation conducting ionomers. They are unique in terms of their chemical stability and transport properties, however, typical in terms of their salt-like brittleness in the dry state and extensive swelling at high humidity and in water. To make the unique properties of sPPS available in application, the goal of this work is to take a comprehensive approach to their viscoelastic reinforcement. Therefore, the structure of this thesis entails three related aspects along the process from pure materials to the optimization of robust PEMs for application. The first chapter focuses on the optimization of the intrinsic viscoelastic properties of a particularly suited sPPS (termed S360, with IEC 2.78 meq g-1, EW 360 g mol-1) which lays the groundwork for reliable and systematic further development. To achieve this, relevant properties of S360 are first characterized and viscoelastic shortcomings as seen in water uptake measurements and tensile tests under dry conditions (≤ 30% relative humidity, RH) discussed. The step-growth polymerization of S360 is optimized after finding significant inorganic contamination retained in the established purification process of the widely used monomer sulfonated difluorodiphenyl sulfone (sDFDPS), allowing for the preparation of the ionomer in reproducible high molecular weight. Relevant properties of high molecular weight S360 are characterized and an enhancement of mechanical properties at 30% RH as well as when submerged in water is found. Access to reproducible high quality of S360 enables its first-time use and study as a PEM in a completely fluorine-free WE cell. At 80 °C, record performance amongst fluorine free electrolytes in PEMWEs of 3.48 A cm-2 at 1.8 V is achieved, showcasing the potential of sPPS for application. The second chapter entails the identification and better understanding of a suitable and versatile reinforcement concept for creating robust membranes based on sPPS. To achieve this, the established homogeneously miscible acid-base polymer blends of sulfonated ionomers with poly(benzimidazole) (PBI, and its derivatives PBIO and PBIOO) are discussed in-depth and chosen for later systematic optimization in combination with sPPS. Since the origin of miscibility in PBI blends with sulfonated ionomers is insufficiently described in literature and could facilitate targeted design of new blend components, a model acid-base polymer blend system comprising pyridine-functionalized poly(sulfone) (PSU) is created. Pyridine groups of different basicity tethered to PSU in varying concentration are used to investigate the effect that interpolymer acid-base interaction strength and concentration have on miscibility in blends with 80 wt% S360, as derived from the blend membranes’ cross-sectional SEMs. High mutual compatibility is achieved at high concentration of weak interpolymer interaction, which is interpreted with regards to the observed miscibility in PBI blends. Based on the derived role that hydrogen bonds may play in PBI blends, the difference of interpolymer interaction in solution (during membrane formation) and in the dry membrane is described. This could enable the development of new blend concepts in the future. An exemplary miscible blend that comprises interpolymer hydrogen bonds only in solution but not in the final membrane is shown. The third chapter describes the optimization and balance of properties in the previously described polymer blends with PBIO, following the goal to prepare membranes which can be evaluated in fuel cells and fabricated on a wider scale in order to bring the attractive properties of sPPS into application. To achieve this, S360-blend membranes of varying PBIO content are characterized with regard to conductivity and mechanical properties in various conditions. High mechanical robustness is achieved in S360 blends with 30 wt% PBIO but is accompanied by dramatic reduction of conductivity, due to the charge-consuming acid-base interaction. The findings are translated into blends with fully sulfonated sPPS (termed S220, with IEC 4.54 meq g-1, EW 220 g mol-1) which allows for the creation of membranes that combine mechanical toughness with high conductivity at a ratio of 25 wt% PBIO in S220, making the material suited for production on a commercial casting line and fuel cell testing. Membranes based on S360 that comprise 15 wt% PBIO are designated for further studies in PEMWEs, where membrane requirements differ significantly from that in PEMFCs, highlighting the versatility of the reinforcement approach chosen in this work. Finally, first fuel cell tests of thin spray coated PBIO blend membranes are conducted, and initial durability testing of sPPS-based membranes in fuel cells is possible. Overall, the results presented in this work are strongly interrelated which underlines the importance of comprehensiveness in the successful viscoelastic reinforcement of sulfonated poly(phenylene sulfone)s. Ultimately, the blend membranes resulting from this work can be used as a platform for further development of sPPS-based PEMs in the future.Item Open Access Heterodyne sensing of microwaves with a quantum sensor(2021) Meinel, Jonas; Vorobyov, Vadim; Yavkin, Boris; Dasari, Durga; Sumiya, Hitoshi; Onoda, Shinobu; Isoya, Junichi; Wrachtrup, JörgDiamond quantum sensors are sensitive to weak microwave magnetic fields resonant to the spin transitions. However, the spectral resolution in such protocols is ultimately limited by the sensor lifetime. Here, we demonstrate a heterodyne detection method for microwaves (MW) leading to a lifetime independent spectral resolution in the GHz range. We reference the MW signal to a local oscillator by generating the initial superposition state from a coherent source. Experimentally, we achieve a spectral resolution below 1 Hz for a 4 GHz signal far below the sensor lifetime limit of kilohertz. Furthermore, we show control over the interaction of the MW-field with the two-level system by applying dressing fields, pulsed Mollow absorption and Floquet dynamics under strong longitudinal radio frequency drive. While pulsed Mollow absorption leads to improved sensitivity, the Floquet dynamics allow robust control, independent from the system’s resonance frequency. Our work is important for future studies in sensing weak microwave signals in a wide frequency range with high spectral resolution.Item Open Access Quantum Fourier transform for nanoscale quantum sensing(2021) Vorobyov, Vadim; Zaiser, Sebastian; Abt, Nikolas; Meinel, Jonas; Dasari, Durga; Neumann, Philipp; Wrachtrup, JörgThe quantum Fourier transformation (QFT) is a key building block for a whole wealth of quantum algorithms. Despite its proven efficiency, only a few proof-of-principle demonstrations have been reported. Here we utilize QFT to enhance the performance of a quantum sensor. We implement the QFT algorithm in a hybrid quantum register consisting of a nitrogen-vacancy (NV) center electron spin and three nuclear spins. The QFT runs on the nuclear spins and serves to process the sensor - i.e., the NV electron spin signal. Specifically, we show the application of QFT for correlation spectroscopy, where the long correlation time benefits the use of the QFT in gaining maximum precision and dynamic range at the same time. We further point out the ability for demultiplexing the nuclear magnetic resonance (NMR) signals using QFT and demonstrate precision scaling with the number of used qubits. Our results mark the application of a complex quantum algorithm in sensing which is of particular interest for high dynamic range quantum sensing and nanoscale NMR spectroscopy experiments.Item Open Access Thermoelektronische Energiekonvertierung und ihre fundamentalen Grenzen(2021) Wanke, Robin; Mannhart, Jochen (Prof. Dr.)Durch einen thermoelektronischen Generator kann potenziell sehr effizient Wärmeenergie in elektrische Energie umgewandelt werden. Durch Elektronenemission von einem Emitter zu einem Kollektor kann bei richtiger Materialwahl Leistung generiert werden. Die Elektronen müssen durch ein Gitter geführt werden, um Raumladungen zu verhindern, welche den Strom verringern. Dieser Mechanismus wird in dieser Dissertation auf seine grundlegenden Grenzen untersucht. Dabei wird auf alle relevanten Komponenten geschaut und deren fundamentale und physikalischen Eigenschaften untersucht, um daraus technische Lösungen zu erarbeiten.
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